1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly, it relates to an image processing apparatus for forming an image by reproducing density data in units of respective picture elements with a laser beam.
2. Description of the Related Art
In a recording apparatus of a scan recording type, various systems are employed for digitally reproducing variable densities of images. Such digital gradation reproducing methods include a dither method, an intensity modulation system and a pulse duration modulation system. The dither method is adapted to change on-off area rates of picture elements contained in a plurality of picture elements which are processed as a unit, thereby expressing the variable density of an image. The intensity modulation system is adapted to averagely apply laser energy to picture elements while modulating laser emission intensity in a multistep manner with respect to each picture element, thereby changing the variable density. The pulse duration modulation system is adapted to modulate an emission time per unit picture element while maintaining constant laser emission intensity, as disclosed in Japanese Patent Laying-Open Nos. 61-225971 (1986) and 62-116959 (1987), for example.
In the dither method of processing a plurality of picture elements as a unit, a moire noise which is specific to this method results in a deterioration of the image quality.
In the intensity modulation system, it is possible to basically output the variable density in one-to-one correspondence to the as-read image data, thereby attaining high resolution with fine and smooth gradation characteristics. In a highlight portion, however, it is difficult to attain excellent gradation reproducibility due to a requirement for delicate variable-density control. Further, rough graininess of a half tone portion is stressed due to an influence by the backing of an image recording paper.
On the other hand, the pulse duration modulation system does not have the problem of graininess that arises in the intensity modulation system. However, this system generally moves centers of gravity of picture elements with respect to units of two picture elements. When density data of 400 DPI are reproduced, for example, only reproducibility of 200 DPI is substantially attained to reduce resolution. When the pulse duration modulation system is employed in a unit of a single picture element, on the other hand, the laser emission time is considerably reduced in a picture element having low density, leading to insufficient stability of the image quality in view of toner adhesion.